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Bureau of Mines Information Circular/1985 




Research on the Reliability of Mine 
Electrical Equipment: A Status Report 



By Dean H. Ambrose 




UNITED STATES DEPARTMENT OF THE INTERIOR 



75? 

^/NES 75TH A^ 



h, , &JU4A ■' t^l 



Information Circular 9050 



Research on the Reliability of Mine 
Electrical Equipment: A Status Report 



By Dean H. Ambrose 




UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



\ 









Library of Congress Cataloging in Publication Data: 



Ambrose, Dean H 

Research on the reliability of mine electrical equipment. 

(Bureau of Mines information circular ; 9050) 

Bibliography: p. 11-12. 

Supt. of Docs, no.: I 28.27: 9050. 

1. Coal mines and mining— Electric equipment. 2. Electricity in min- 
ing. I. Title. II. Series: Information circular (United States. Bureau of 
Mines) ; 9050. 



TN295.U4 622s [622'. 334] 85-600151 



CONTENTS 



Page 



1 


2 


2 


3 


4 


9 


10 


11 


13 


14 



Abstract 

Introduction 

Reliability terms 

Reliability research program 

Reliability data base 

Application of reliability data 

Conclusions 

References 

Appendix A. — Field data collection technique and components' sample size 

Appendix B. — Field data conversion to failures per unit-year 

ILLUSTRATIONS 

1 . Ground fault protection system 4 

2. Reliability characteristics of circuit breakers using theoretical 

analysis 5 

3. Reliability characteristics of circuit breakers using field data 5 

4. Reliability characteristics of undervoltage relays using theoretical 

analysis 6 

5. Reliability characteristics of solid state undervoltage relays using 

life test data 6 

6. Reliability characteristics of electromechanical undervoltage relays 

using field data 7 

7. Reliability characteristics of ground check monitors using field data 

and maintenance records 7 

TABLES 

1. Molded-case circuit breaker data 4 

2. Undervoltage relay data 5 

3 . Ground check monitor data 7 

4. Splicing rates of trailing cables on mine machines 8 

5. Replacement rates of trailing cables on mine machines 8 

6. Motor data 8 

7. Control circuitry data 8 

8. Failure rate data source comparison 9 

9. Maintenance intervals for mining electrical equipment 10 

10. Probability of hazardous voltages of mining operations 10 

A-l. Components' sample size 13 



^ 



UNIT OF 


MEASURE 


ABBREVIATIONS 


USED 


IN THIS 


REPORT 


pet 


percent 






yr 


year 


V 


volt 











RESEARCH ON THE RELIABILITY OF MINE ELECTRICAL 
EQUIPMENT: A STATUS REPORT 

By Dean H. Ambrose 1 



ABSTRACT 

This Bureau of Mines publication summarizes the status of mine elec- 
trical equipment reliability research. Failure rate data were estimated 
from theoretical, laboratory, and field studies. Tables are presented 
listing reliability characteristics of molded-case circuit breakers, un- 
dervoltage relays, ground check monitors, cables, motors, and control 
circuitry. Application of failure rate data is shown in tables listing 
maintenance scheduling intervals of components and components that are 
most sensitive to the probability of hazardous voltages on power dis- 
tribution systems in the mines. Complete, accurate, and appropriate 
failure rates on mine equipment are of the utmost importance for relia- 
bility evaluations. 



Electrical engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA, 



INTRODUCTION 



In the past, reliable design could be 
described simply as picking good parts 
and using them correctly. Today, the 
complexity of systems and the demand for 
good reliability in many applications 
means that sophisticated methods based on 
numerical analysis and probability tech- 
niques have been accepted on determining 
the feasibility (from the reliability 
standpoint) of systems. Classic examples 
of ultrahigh reliability exist in systems 
built for the military and the National 
Aeronautics and Space Administration 
(NASA). System availability and time 
needed for maintenance are also important 
to many systems. 

Electrical systems developed for mech- 
anized underground coal mines over the 
past 25 yr have been well accepted by 
the mining industry. Most people agree 
that improving mining equipment reliabil- 
ity, at least from the design viewpoint, 
is not an easy task. Despite the need 
for higher reliability, electrical sys- 
tems of today are deteriorating faster 
than necessary because of the poor qual- 
ity of maintenance provided during the 
life of the system (l). 2 Federal Regula- 
tions (30 CFR 75 and 77) mandate timely 



electrical preventive maintenance — 
equipment examination, testing, and main- 
tenance. The preventive maintenance pro- 
grams at most mines are generally hap- 
hazard and neglected (_2, p. 455). 

A nationwide survey of electrical 
equipment reliability, completed by the 
Institute of Electrical and Electronic 
Engineers (IEEE) in industrial plants 
such as chemical, petroleum, cement, 
etc. , revealed that inadequate mainte- 
nance is a significant cause of equipment 
failures (3, p. 268). In response to 
this survey, the Bureau of Mines funded 
several studies to address the problems 
relating to inadequate maintenance; these 
studies were aimed at increasing produc- 
tion and improving mine safety by im- 
proving mine electrical equipment 
reliability. 

In a series of investigations, the Bu- 
reau gathered and analyzed failure rate 
data for mine electrical equipment. This 
report discusses the reliability data ob- 
tained from these studies, the most im- 
portant applications of this new informa- 
tion, and future research that would 
build upon the present reliability pro- 
gram results. 



RELIABILITY TERMS 



Availability . — Capability of an item, 
considering its reliability and mainte- 
nance, to perform its required function 
at a stated instant in time (4^, p. 333). 

Confidence interval . — The range of val- 
ues within an estimate of the failure 
rate that will be supported (will not be 
contradicted) by the data (_5, pp. 16-19). 
In this paper the confidence interval of 
A has the property: lower limit < A < 
upper limit > 0.95; i.e., the probability 
that the confidence interval covers or 
contains the true value of A is at least 
95 pet. 



2 Under lined numbers in parentheses re- 
fer to items in the list of references 
preceding the appendixes. 



Failure . — The termination of an item's 
capability to perform its required func- 
tion (4, p. 334). 

Failure rate (A) . — The rate at which 
failures occur in a certain time inter- 
val; i.e., failure per unit-time. The 
time units may be hours, years, shifts, 
etc. (_5, p. 11). 

Hazard rate . — The instantaneous failure 
rate; the probability that a component 
will fail in a small interval of time (_5, 
p. 11). 

Maintainability . — A characteristic of 
design and installation expressed as the 
probability that an item will be retained 
in or restored to a specific condition 
within a given period of time, provided 
maintenance is performed in accordance 
with prescribed procedures and resources 
(4, p. 335). 



Mean time between failures. — For a par- 
ticular interval, the total functioning 
life of a population of an item divided 
by the total number of failures within 
the population during the measurement 
period (4, p. 336). 



Reliability . — The characteristic of an 
item expressed by the probability that it 
will perform a required function under 
stated condition for a stated period of 
time (4, p. 337). 



RELIABILITY RESEARCH PROGRAM 



The failure rates of mine electrical 
power system components for coal mine op- 
erating and environmental conditions were 
estimated using one or more of the fol- 
lowing: 

1. A theoretical model based on a 
thorough understanding of the physics of 
failure mechanisms was used to predict a 
failure rate. 

2. A statistical estimate of the fail- 
ure rate was made using accumulated field 
data on failure or information from main- 
tenance records. This technique is wide- 
ly used in other industries. 

3. Accelerated life tests were per- 
formed by conducting magnified load tests 
in a laboratory environment. In this 
type of accelerated test, each sample was 
tested at higher than normal stress lev- 
el. It is possible to obtain the life 
distribution of the devices and their 
failure rates from the data collected by 
performing these tests. This technique 
is useful in obtaining a rough estimate 
of the failure rate in a very short 
time. 

Technique (2) (statistical estimation 
using field data) is by far the best and 
most accepted approach to obtain a real- 
istic failure rate for a component (4, p. 
37). The field data collection technique 
and components' sample size are given in 
appendix A. 

The selection of components for relia- 
bility evaluation were protective devices 
and certain mine power equipment. The 
protective components of the mine power 
distribution systems are typically the 
molded-case circuit breaker, the under- 
voltage relay (UVR) , the ground check 
monitor (GCM), and the ground-fault relay 
(GFR). (See figure 1.) These components 



are responsible for disconnecting power 
circuits to mining equipment under abnor- 
mal system conditions, such as circuit 
overcurrents , ground faults, system un- 
dervoltages, or open ground conductors. 
To ensure the protection of personnel and 
equipment, abnormal conditions must be 
detected and cleared promptly. This re- 
quirement implies that protective compo- 
nents must be extremely reliable. A high 
degree of reliability also assures in- 
creased productivity by maximizing the 
availability of the equipment. The four 
possible undesirable modes of operation 
of protection devices are — 

(a) failure to trip when desired, 

(b) undesirable tripping, 

(c) failure to close, and 

(d) catastrophic failure. 

The first mode of failure is the most 
significant from the safety point of 
view, while the other three modes affect 
productivity (6^, p. 97-98). 

The other mine power equipment investi- 
gated were cables, motors, motor start- 
ers, and control gear. A cable can fail 
and result in electrical hazards to per- 
sonnel, an ignition source for fires, or 
loss of production due to any one the 
following causes: 

(a) Any one of the three phase conduc- 
tors or the ground conductor or the 
ground-check conductor can break open; 

(b) short circuits due to insulation 
failure can occur; 

(c) shielding of phase conductors can 
fail; and 

(d) jackets can fail. 

The causes of a motor's failure are nu- 
merous. However, mechanical and electri- 
cal failures and starter-control gear 
failures can result in loss of production 
and equipment, and in electrical hazards 
to personnel and mine fires (6, p. 152). 



480 V 
3 phase 



J 



War 



480V/I20V ™gg^. 



Mine power center 



Mine face equipment 



Breaker 



UVR 



#4H 

GFR GCM 



GFR- 



rOi 



imor 



*\ 



$< 



Filter 




£-£ 



Filter 



FIGURE 1. - Ground fault protection system. 



RELIABILITY DATA BASE 



Table 1 summarizes the results obtained 
from theoretical analysis, accelerated 
life tests, and field data evaluation of 
molded-case circuit breakers. The relia- 
bility characteristics obtained from the 
theoretical analysis (fig. 2) show that, 
for the breaker to have 95-pct reliabil- 
ity, it must be removed from service be- 
fore 1 yr of continuous service. Field 
data (fig. 3) also indicate a removal 
time of less than a year for greater than 
95-pct reliability. Replacement cost 
would be an expensive item for the mine 
operators. With good maintenance proce- 
dures, such as periodic inspection, cor- 
rect examination, and calibration re- 
quirements, the circuit breakers could 
have >95-pct reliability for a longer 
service life (_5, p. 74). 

Table 2 summarizes results obtained 
from theoretical analysis, accelerated 
life tests, and field data evaluation of 
both electromechanical (EM) and solid 



TABLE 1. - Molded-case circuit breaker 
data 





Theo- 


Lab 


Field 




retical 


test 


data 


Failure rate ( A) 








failues per 








year. . 


1 0.1607 


2 0.30 


3 0.12 


95-pct confidence 








interval of A: 










l 0. 05975 
1 0.3084 


ND 
ND 


'♦o.oee 




"+0.204 


5 MTBF (1/A)...yr.. 


4 6.22 


^3.33 


^8.33 



ND No data 

Wenkata (_5, p. 75); extrapolated (aver- 
aged). 

2 Hill and Collins (_7, p. 17). 

3 Hill and Collins (7, p. 16). 

^Calculated. 

5 Mean time between failures. 

state (SS) UVR's. Theoretical part fail- 
ure rates were extrapolated from rates 



uu 


K ' ' ' ' 


i 


' I 


1 




V> N 










\\ v 




KEY 






\ > N. 








90 


-1 \ v 


& 


Upper limit 


— 




\ « > 


o 


Failure rate 






- \ °\ X 


• 


Lower limit 




80 




\ 




- 


70 


\ 


V 


v 


~ 




V \ 




*>. 


. 




\ ^ 




v 




s: 


\ ° 

\ \ 
\ \ 

\ \ 

A ^ 




V 


ti 


50 


\ \ 
\ x 
\ \ 
\ \ 

\ N 






- 


40 
















V 




30 








" 




















20 








-6 


10 


i i i i 


1 


1 


i 



4 6 

TINIE,yr 



FIGURE 2. - Reliability characteristics of cir- 
cuit breakers using theoretical analysis 



<^ 60 
x* 

UJ 

a 



> 50 



10- 



KEY 
a Upper limit 
o Failure rate 
• Lower limit 




4 6 

TIME.yr 



10 



FIGURE 3. - Reliability characteristics of cir- 
cuit breakers using field data. 



TABLE 2. - Undervoltage relay (UVR) data 





Theoretical 


Life test 


Field 




EM 


SS 


EM 


SS 


EM 


SS 


Failure rate ( A) 

failures per year. . 
95-pct confidence interval of A: 


1 0. 15 

4 0.0498 

'♦0.159 

4 6.67 


io.ne 

ND 

ND 

1+ 7.35 


ND 

ND 
ND 
ND 


2 0.49 

2 0.36 
2 0.73 

2 2.04 


3 0.13 

t+ 0.0715 

'♦0.221 

4 7.7 


ND 

ND 
ND 




ND 



EM Electromechanical. SS Solid state. ND No data. 

^-Venkata (_5, p. 85); extrapolated (summed). 

2 Collins (8, p. 61). 

3 Hill and Collins (_7, p. 16). 

H Calculated. 

5 Mean time between failures. 



tabulated by the United Kingdom's Atomic 
Energy Agency (UKAEA). A comparative 
analysis shows that the SS UVR performs 
(theoretically) better than the EM UVR. 
One reason might be that the SS UVR has 



fewer mechanical components. However, as 

shown in figure 4, both UVR's degrade to 

the 95-pct reliability levels within 6 to 
18 months of operation. 



100 



100 



90 



80 



70- 



x 
w 

Q 



60 



^50- 



< 



40- 



30 



20 



iv — r | i | i | i i i 
>. N « N KEY 


\\ \ a EM upper limit 
— \ ^ N O EM failure rate _ 


\ c\ »^ • EM lower limit 


\ \- \ # A SS failure rate 
\ \\ \ 


A \ A ^* 




\ " \ v - 


\ % *\ ^ 


— \ \ •. *x — 


\ ^ ^ 


\ \A ^ 


\ Q\ ^ " 


\ \\ ^J' 


A "\ v 


\ V\ 


\ W 


\ v - \ 


X °^ x * 


\ \ A 


\ * X 


\ °^ ^ 


\ v A 


\ > ^ 


\ x cx v 


\ ■> A^ 


\ ^ X - 


\ VV " 


\ X °0> 




\ ^ N 


1 . 1 1 1 ■ 1 



2 4 6 8 

TIME.yr 

FIGURE 4. - Reliability characteristics of under- 
voltage relays using theoretical analysis. 



x 

UJ 
Q 



CD 
< 



UJ 

or 



KEY 
A SS upper limit 
o SS failure rate 
• SS lower limit 




4 6 

TIME,yr 



"I 

-A 1 



8 



=8 
10 



FIGURE 5. - Reliability characteristics of solid 
state undervoltage relays using life test data. 



Life test data (fig. 5) showed a de- 
gradation of 95-pct reliability of SS UVR 
within 2 months of continuous service, 
while the failure rate determined from 
field data for an EM UVR was less than 
the failure rate predicted in a theoreti- 
cal analysis. This suggests that the 
part failure rates used for the predic- 
tions were not too pessimistic. As shown 
in figure 6, the EM UVR falls below the 
95-pct reliability level after 3 to 8 
months of continuous service. This means 
that if the UVR's were to be extremely 
reliable, they would need replacement be- 
fore completion of a continuous service 
of 1 yr. The level of reliability of UVR 
could be raised about 95 pet with suit- 
able modifications in the design or main- 
tenance of the relay (8, p. 84). 



Table 3 shows reliability characteris- 
tics of GCM's based on field data and 
maintenance records. Both show the reli- 
ability fall below the 95-pct level with- 
in 2 months of continuous service (fig. 
7). The lower limit of the confidence in- 
terval for field is of the same magnitude 
of failure rate established from the 
maintenance records. 

Trailing cables feeding shuttle cars 
and continuous miners in underground coal 
mines regularly are subjected to harsh 
treatment. The failure frequency and the 
associated splicing rates are very high 
compared with the failure rate of compo- 
nents such as circuit breakers and UVR's. 
Table 4 shows splicing rates on trailing 
cables feeding shuttle cars and continu- 
ous miners, as estimated from field data. 



100 



90- 



80- 



701- 



. 60 h 
x 

_ 

z 

>- 501— 



40- 



30- 



20- 



10- 



■ , 1 1 , 1 , 1 


\\y KEY 


_\ * \ £ EM upper limit - 


\ x \ 

\ ° v » o EM failure rate 


\ \ \ • EM lower limit 


\ i \ 


- V \ ^ 


A V N 


\ ° S- 


\ N • 


\ « N 


\ ' \ 


— \ » • 


\ v \ 


\ Q . N 


V \ •- 


\ v \ 


\ v * ^ 


\ \ - 


\ ^ ^ 


\ *•» 


\ °v *V 


\ \ N> 


\ \ 


\ N 


\ °N 




\ V 


\ X 


\ X 










\ "° % x 




7\. ** 




N. O v _ 








X T 


^v 


^■^^ 


f , 1 



4 6 

TIME,yr 



10 




KEY 
& Field upper limit 
O Field failure rate 
• Field lower limit 
a Records failure rate 



gj n— ■&. 



FIGURE 6. • Reliability characteristics of electro- 
mechanical undervoltage relays using field data. 



2 3 

TIME,yr 

FIGURE 7. - Reliabil ity characteristics of ground check 
monitors using field data and maintenance records. 



TABLE 3. - Ground check monitor (GCM) data 





Theo- 


Lab 




Mainte- 




reti- 


test 


Field 


nance 




cal 






records 


Failure rate ( A) 










failures per year. . 


ND 


ND 


11.6 


2 0.75 


95-pct confidence interval of A: 












ND 


ND 


3 0.93 


ND 




ND 
ND 


ND 
ND 


3 2.72 
3 0.625 


ND 


"MTBF (1/ A) yr. . 


3 1.33 



ND No data. 

-Hill and Collins (7, p. 16). 

z Hill and Collins (_7, p. 17). 

'Calculated. 

u Mean time between failures. 



Each cable failure resulting in the need insulation failure with a safety hazard 
for a splice was considered to be a cable involved. 



A number of field data samples were 
isolated from the above data, in terms of 
the time interval in shifts, starting 
from a new or rebuilt cable until it was 
replaced. Table 5 shows the replacement 
rate for shuttle cars and continuous 
miners. Comparing the mean time between 
replacement of 96.6 shifts for trailing 
cable on a shuttle car with the mean time 
between splice of 3.01 shifts reveals 
that over 30 splices are commonly made 
before a cable is replaced in the partic- 
ular mine under study. 

A large number of ac and dc motors are 
used on underground machines. The motors 
suffer frequent failures (electrical or 
mechanical) with resultant safety hazards 
and downtime. The failure rate in table 
6 represents all motors used in the mine, 
irrespective of size, location, and ap- 
plication. Different application of mo- 
tors in the mine are expected to generate 
different failure rates. 

Failure rates of control circuitry used 
for continuous miners and shuttle cars 
are shown in table 7. The control cir- 
cuitry includes the various contactors, 
motor starters, switches, and the general 
wiring on individual machines. Consider- 
ing the complexity of the control cir- 
cuitry and available data, it was not 
possible to estimate individual failure 
rates of the various components of the 
control circuit. 



TABLE 4. - Splicing rates of trailing 
cables on mine machines 



TABLE 5. - Replacement rates of trailing 
cables on mine machines 



Splice rate ( A) 

splice per shift. . 
95-pct confidence in- 
terval of A: 

Lower 

Upper 

4 MTBS (1/ A).... shifts.. 



Shuttle 
car 



!0.332 



L 0.292 
3 3.01 



Contin- 
uous 
miner 



2 0.072 



3 0.0504 

3 0.1022 

3 13.9 



1 Venkata (_5, p. 102). 

2 Venkata (_5, p. 106). 

Calculated. 

^Mean time between splices 



Replacement rate ( A) 

replacements per shift. . 
95-pct confidence in- 
terval of A: 

Lower 

Upper 

*MTBR (1/A) shifts.. 



Shuttle 


Contin- 


car 


uous 




miner 


0.01035 


0.00714 


0.005 


0.0023 


0.0176 


0.0146 


96.6 


140.1 



Mean time between replacement. 



Source: Venkata (5, p. 107). 



TABLE 6. - Motor data 



1 Failure rate ( A) 

failures per shift. . 
2 95-pct confidence in- 
terval of A: 

Lower 

Upper 

3 MTBF (1/A) shifts.. 



ac 
motor 



0.056 



0.00381 

0.02436 

2 17.86 



dc 
motor 



0.0379 



0.0234 

0.06064 

2 26.39 



1 Venkata (_5, p. HO). 

2 Calculated. 

3 Mean time between failures. 

TABLE 7. - Control circuitry data 





Shuttle 


Contin- 




car 


uous 
miner 


Failure rate ( A) 






failures per shift. . 


1 0.022 


2 0.32 


3 95-pct confidence in- 






terval of A: 








0.01815 
0.0561 


0.0176 




0.0545 


l+ MTBF (1/A) shifts.. 


3 45.5 


3 31.3 



Venkata (_5, p. 110). 

2 Venkata (_5, p. 111). 

Calculated. 

4 Mean time between failures. 

No failure data are available for such 
equipment as transformers, cable cou- 
plers, contactors, and other items us- 
ually found in the mine power system. 



However, some failure rate data are 
available from IEEE (_9, pp. 17-41) and 
UKAEA ( 10 , p. 72) , and suitable weighting 
or multiplication factors could be appl- 
ied to certain mine operating and envi- 
ronmental conditions (_5, pp. 111-114). 
Table 8 lists the failure rates, convert- 
ed to a common unit — failures per unit- 
year — for many electrical components and 
gives comparative data from IEEE and 
UKAEA, where such data exist. Although 



several components can be compared using 
these data, no strong conclusion can be 
made concerning any general multiplica- 
tion factors that would be applicable to 
the mine environment, largely because of 
the unavailability of comparable data for 
many of the components. The conversion 
processes for presenting research data in 
failures per unit-year are shown in ap- 
pendix B. 



APPLICATION OF RELIABILITY DATA 



Preventive maintenance traditionally 
means performing various tasks, on a pe- 
riodic basis, to extend the life of a 
component. An example is the periodic 
lubrication of motor bearings. However, 
a more general definition is the process 
of prolonging the useful life for a com- 
ponent, and when failure is imminent, re- 
placing the component with minimum down 
time and personal hazard. 

Problems and concerns of implementing a 
preventive maintenance program could be 
discussed at great length. Preventive 
maintenance is the key to improved pro- 
ductivity and safety in mines (2_, p. 
488). While the mining systems pose 



unique problems, these difficulties are 
no more severe than those of other 
industries. 

Research (_5, pp. 10-42) determines 
optimal maintenance schedules for certain 
mine power equipment. The information 
used to obtain the results is based upon 
failure rate from field data and es- 
timation from other sources, discussions 
with mine operators, and engineering 
judgment. The preventive maintenance 
intervals for various mining electrical 
equipment are shown in table 9. These 
intervals seek to maintain a 95-pct level 
of reliability. 



TABLE 8. - Failure rate data source comparison 



Failure rate, failures 
per unit-year 



Research 



UKAEA 



IEEE 



Molded-case circuit breaker. . 

Undervoltage relay (UVR) 

Ground check monitor (GCM)... 

Ground fault relay 

Cable splicing: 

Shuttle car 

Continuous miner 

Cable replacement: 

Shuttle car 

Continuous miner 

ac motors 

dc motors 

Machine control circuitry: 

Shuttle car 

Continuous miner 

ND No data. 



0.0086 

.0093 

.1143 

None 

35.174 
12.708 

1.096 
1.513 
1.059 
.5918 

5.825 
5.648 



0.0175 

.0438 

ND 

ND 

ND 
ND 

ND 

ND 

.0876 

.0876 

ND 
ND 



0.0052 
ND 
ND 
ND 

ND 

ND 

ND 

ND 

.0109 

.0556 

ND 

ND 



10 



Research ( 11 , pp. 106-156) developed 
reliability models for the electric power 
systems of surface and underground mines 
capable of predicting the probability of 
electrical shock hazards existing on dis- 
tribution systems in the mines. Relia- 
bility data are used to predict hazard 
rates through fault-tree analysis. 

Predictions of mine electrical accident 
probabilities using reliability modeling 
techniques have produced several impor- 
tant results. Table 10 summarizes the 
results for the various mining operations 
analyzed. The most sensitive components 
are defined as the components which, if 
reliability is increased, will provide 
better protection against electrical 
shock hazards. 



TABLE 9. - Maintenance intervals 1 for 
mining electrical equipment, months 

Cables: 

Shuttle car , 2.5 

Continuous miner ^ c 

Roof bolter 5^0 

Couplers: Low-voltage 36.0 

Molded-case circuit breaker 7.0 

Motors: 

ac motors 12.5 

dc motors 21.5 

iThe following equipment is subject to 
emergency repairs only: 

Ground check monitors 

Ground fault relays 

High-voltage couplers 

Power center transformers 



CONCLUSIONS 



The mine electrical reliability re- 
search program evaluated molded-case cir- 
cuit breakers, undervoltage relays, 
ground-check monitors, cables, motors, 
and control circuitry. Failure rates 
were estimated from theoretical, labora- 
tory, and field studies. Results showed 
that to maintain 95-pct reliability, many 



components would have to be removed from 
continuous service within 2 months to 1.5 
yr. Failure rate data were used to cal- 
culate optimal maintenance scheduling on 
various mining equipment. For example, 
the optimal maintenance interval is 7 
months for circuit breakers and 12.5 
months for ac motors; whereas ground- 



TABLE 10. - Probability of hazardous voltages of mining operations 



Operation 


Probability of 
hazardous voltage, 
hazards per year 


Most sensitive 
component 


Underground coal mine: 


0.0047 
.1871 

.01403 

.005946 

.00003 

.00153 

.00409 
.00003 

.02897 


Ground diode. 


Underground M/NM mine: 
480-V system 


Safety ground system. 

Open or short grounding resistor. 
Cable insulation and leakage detector. 
Open ground wire. 

Ground check monitor and grounding 

resistor. 
Ground wire. 


300-V system 


120-V system 


Dredging: 

2, 300-V system 




120-V system 


Open ground wire. 

Grounding resistor and overhead ground 
wire. 


Open pit M/NM mine: 

Distribution system.... 



M/NM Metal-nomental. 



Source: Hill and Collins (11, pp. 123-147). 



11 



fault relays require emergency repair 
only. Prediction of mine electrical ac- 
cident probabilities using reliability 
modeling techniques has produced several 
important results to improve mine dis- 
tribution systems for various mining op- 
erations. The most sensitive components 
to probability of hazardous voltage for 
underground coal mine operations are the 
ground diodes for shuttle cars and the 
safety ground systems for continuous 
miners. 

The need for complete, accurate, and 
appropriate failure rates for mine compo- 
nents is of the utmost importance. Sev- 
eral of the failure rates used for appli- 
cation purposes are extrapolations of 
failure rates for similar devices from 
other sources. The failure rates for the 
same devices in a less hostile environ- 
ment than a mine are also suspect. These 
facts point out the importance of com- 
plete data from field tests and mainte- 
nance records. Complete data on failure 
rates of mine electrical equipment should 
include components installed in an oper- 
ating coal mine power system. These com- 
ponents include ground-check monitors, 
ground-fault relays, molded-case circuit 
breakers, electromechanical and solid 
state undervoltage relays, ac and dc 
motors, control circuitry, cables, cable 
couplers, and transformers. 

A suggested field data collection tech- 
nique could include the following: 

1. Select several underground coal 
mine sites to represent the worst mine 
environment. Experience from previous 
in-mine tests shows that working sections 
are phased out periodically; therefore, 
several mines would be needed to provide, 
on a continuous basis, an adequate compo- 
nent sample size for failure rate data 
analysis. 

2. All electrical components identi- 
fied for evaluation would be tagged with 
unique identification stickers. 



3. Maintenance records would be main- 
tained for each tagged component. 

4. Postcards would be provided to the 
mine personnel for notification of in- 
stallation and failure dates for each 
component. 

5. As tagged components fail they 
would be removed from service by the mine 
personnel and returned for laboratory 
analysis to determine failure modes and 
possible failure mechanism. For larger 
components, e.g., cables, motors, and 
transformers, mine personnel would pro- 
vide their best judgment on the cause of 
component failure. 

Failure data collection and failure 
rate estimation has only begun to help 
achieve mine power system reliability. 
After complete data on failure rates are 
assembled, the overall system reliabil- 
ity, availability, maintainability, and 
safety (RAMS) models could h~ developed 
and quantitative results can be obtain- 
ed. Suitable component replacement, 
preventive maintenance, and optimum 
maintenance-crew deployments can be 
achieved to maximize both safety and pro- 
ductivity of the mine. 

Long-range findings of the RAMS studies 
of existing systems and equipment can be 
used in the design of future systems and 
equipment, particularly because large- 
scale automated or semi automated systems 
of mining are evolving for use in the 
near future. Therefore, a high degree of 
RAMS is an essential attribute of these 
systems to justify the cost and complex- 
ity of such automation. The system must 
always be designed to be fail-safe, yet 
cost-effective. These aspects cannot be 
overemphasized, as already proven by re- 
cent advances in space and other sophis- 
ticated technologies. Coal mining, 
though it looks like a mundane applicat- 
ion, is an equally challenging task. 



REFERENCES 



1. Syska and Hennessy, Inc. Engineer- 
ing Management Series. V. 6. Applied 
Preventive Maintenance. Eng. Manage. 
Div. , New York, 1981, 49 pp. 



2. Morely, L. A. Mine Power Systems. 
Volume II (contract J0155009, PA St. 
Univ.). BuMines OFR 178(2)-82, 1980, 532 
pp.; NTIS PB 83-120386. 



12 



3. Heising, C. R. Quantitative Rela- 
tionships Between Scheduled Electrical 
Preventive Maintenance and Failure Rate 
of Electrical Equipment. IEEE Trans. 
Ind. Appl. , May/June 1982, 220 pp. 

4. Reliability Analysis Center (RADC 
Griff is Air Force Base, New York). Reli- 
ability Design Handbook. Cat. No. RDH- 
376, Mar. 1976, 342 pp. 

5. Venkata, S. S., M. Chinnarao, E. W. 
Collins, and E. U. Ibok. Transients Pro- 
tection, Reliability Investigation, and 
Safety Testing of Mine Electrical Power 
Systems. Volume II. Reliability of Mine 
Electrical Power Systems (grant G0144137, 
WV Univ.). BuMines OFR 52-78, 1979, 197 
pp.; NTIS PB 81-166779. 

6. Stanek, E. K. Digital Computation 
of Transients and Safety Testing of Mine 
Electrical Power Systems (grant G0144137, 
WV Univ.). BuMines OFR 52-78, 1977, 200 
pp. NTIS PB 283400/AS. 

7. Hill, H. W. , and E. W. Collins. 
Mine Power System Safety and Reliability 



Improvement. Volume II (contract 
J0199119, WV Univ.). BuMines OFR 183-84. 
1983. 19 pp.; NTIS PB 85-109288. 

8. Collins, E. W. Reliability Analy- 
sis, Maintenance Scheduling of Solid 
State Undervoltage Relay. M.S. Thesis, 
WV Univ., Morgantown, WV, 1980, 123 pp. 

9. Institute of Electrical and Elec- 
tronics Engineers. IEEE Recommended 
Practice for Design of Reliable Indus- 
trial and Commercial Power Systems. IEEE 
Standard 493-1980. Wiley, 1980, 224 pp. 

10. Green, A. E. , and A. J. Brourne. 
Reliability Technology. Wiley, 1972, 233 
pp. 

11. Hill, H. W. , and 
Mine Power System Safety 
Improvement. Volume 



E. W. Collins. 

and Reliability 

I (contract 



J0199119, WV Univ.). BuMines OFR 46-85, 
1981, 186 pp. 

12. Stanek, E. K. Enhancement of Mine 
Power System Safety and Reliability 
(grant G0188097, WV Univ.). BuMines OFR 
116-80, 1979 207 pp.; NTIS PB 81-125361. 



13 



APPENDIX A.— FIELD DATA COLLECTION TECHNIQUE AND COMPONENTS* SAMPLE SIZE 



A. Data collection technique for the 
molded-case circuit breakers with UVR's, 
GFR's, and GCM's: 

One underground coal mine agreed to 
cooperate with a Bureau contractor 
in providing an operation mine site 
for field testing. The components 
identified were tagged with unique 
identification stickers. Postcards 
were provided to the mine personnel 
for notification to contractor of 
installation and failure dates for 
each component. As tagged compo- 
nents failed, they were removed 
from service by the mine personnel. 
No GFR units failed. Components 
involved in field testing were 
phased into operational service on 
a new section power center from May 
1981 to September 1983. 

B. Collection technique for trailing 
cables, motors, and control circuitry: 

One underground coal mine agreed to 
provide the Bureau contractor with 
maintenance foremen's reports for a 



1-yr period. The data available 
over the year contained the dates 
and shifts during which trailing 
cables, motors, or control cir- 
cuitry were repaired or replaced. 
C. Table A-l lists components' sample 
sizes. 

TABLE A-l. - Components' sample size 

Molded-case circuit breaker 14 

Undervoltage relay (UVR) 14 

Ground check monitor GCM) 14 

Ground fault relay (GFR) 14 

Cable splicing: 

Shuttle car 10 

Continuous miner 6 

Cable replacement: 

Shuttle car 10 

Continuous miner 5 

ac motors 56 

dc motors 68 

Machine control circuitry: 

Shuttle car 6 

Continuous miner 4 



14 



APPENDIX B.— FIELD DATA CONVERSION TO FAILURES PER UNIT-YEAR 



To obtain failures 
determine the numbe 
year, then divide by 
For some components, 
year proceeded duri 
the total shifts (1 
in this case, it is 
the number of shifts 
number of shifts pe 
fraction of a year o 
to divide this numbe 
failures to obtain f 



per unit-year, first 
rs of failures per 

the number of units. 

operation during the 
ng only a portion of 
,059) during a year; 

necessary to divide 

in operation by the 
r year to obtain the 
f operation, and then 
r into the number of 
ailures per year. 



Molded-case circuit breakers: 

Failures per year.. 0.12 

Units v 14 

Failures per unit-year 0.0086 

Undervoltage relays (UVR's): 

Failures per year 0.13 

Units t 14 

Failures per unit-year 0.0093 

Ground fault monitors (GFM's): 

Failures per year 1.6 

Units t 14 

Failures per unit-year 0.1143 

Cable splicing — Shuttle cars: 

Failures 93 

Fraction of year in operation 
= (280 shifts v 1,059 shifts 

per year) v 0.2644 

Failures per year 351.74 

Units t 10 

Failures per unit-year.. 35.174 

Cable splicing — Continuous miners: 

Failures 36 

Fraction of year in operation 
= (500 shifts v 1,059 shifts 

per year) ^ 0.4721 

Failures per year 76.255 

Units v 6 

Failures per unit-year.. 12.708 

Cable replacement — Shuttle cars: 

Failures 10 

Fraction of year in operation 
= (966 shifts f 1,059 shifts 

per year) -s- 0.9122 

Failures per year 10.963 

Units v 10 

Failures per unit-year.... 1.096 

*U.S. CPO: 1985-505-019/20,110 



Cable replacement — Continuous miners: 

Failures 5 

Fraction of year in operation 
= (700 shifts x 1,059 shifts 

per year) i 0.6610 

Failures per year 7.564 

Units v 5 

Failures per unit-year.. 1.513 

ac motors: 

Failures 28 

Fraction of year in operation 
= (500 shifts v 1,059 shifts 

per year) -5- 0.4721 

Failures per year 59.309 

Units 7 56 

Failures per unit-year.. 1.059 

dc motors: 

Failures 19 

Fraction of year in operation 
= (500 shifts t 1,059 shifts 

per year) v 0.4721 

Failures per year 40.246 

Units v 68 

Failures per unit-year.. 0.5918 

Control circuitry — Shuttle cars: 

Failures 16 

Fraction of year in operation 
= (500 shifts t 1,059 shifts 

per year) v 0.4721 

Failures per year 33.891 

Uni t s -i- 6 

Failures per unit-year.. 5.648 

Control circuitry — Continuous miners: 

Failures 11 

Fraction of year in operation 
= (500 shifts t 1,059 shifts 

per year) -r 0.4721 

Failures per year 23.300 

Units -5- 4 

Failures per unit-year.. 5.825 



INT.-BU.O F MINES, PGH..P A. 28112 






U.S. Department of the Interior 
Bureau of Mines— Prod, and Distr. 
Cochrans Mill Road 
P.O. Box 18070 
Pittsburgh, Pa. 15236 



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